Organization and Trends of the Periodic Table
Metals, non-metals and metalloids The main division of the elements of the periodic table is into metals and non-metals. Most of the elements are metals and they share physical properties that include conduction of heat and electricity, malleability, ductility and lustrous appearance. They are located in center of the table. There are only a few non-metals that appear in the upper-right hand corner of the table, above the darkened line that appears on most tables. Nonmetals do not have the physical properties that can be associated with metals. The line dividing metals and nonmetals is only an approximation because this darkened line designates the location of the metalloids, which are sometimes referred to as semi-metals. These elements possess properties that are both metallic and nonmetallic. Groups or Families Elements of the periodic table that are along the same vertical column are said to belong to the same group or family, having silimar chemical properties. The main groups of the periodic table, from left to right, are alkali metals (Group 1A) and alkaline earth metals (Group 2A). Group 3A, beginning with Boron (B), through to Group 6A inclusive, do not have specific group names. Halogens designate the elements of Group 7A and noble gases are Group 8A elements. The single exception is Hydrogen. Although it is located in Group 1A, it is not an alkali metal, but a diatomic gas that sometimes behaves like halogens. The elements within all these groups are considered main-group'' or ''representative elements''.' The block of metals in between Group 2A to Group 3A do not have distinct group names or numbers but are collectively referred to as '''transition metals. The group number is an indication of the number of valence electrons (outermost electrons) for each element in that group. All the elements in the first column or group all have a single valence electron (H, Li, Na, K, etc.). The second column elements all have 2 valence electrons (Be, Mg, Ca, Sr, etc.). Skipping over the transition metals, to Group 3A elements, which all have 3 valence electrons (B, Al, Ga, etc.). The elements in the next column (C, Si, Ge, etc.) all have 4 valence electrons. The elements in the next group (N, P, As, etc.) have 5 valence electrons. O, S, Se, and the others in this column have 6 valence electrons. The halogens in Group 7A (F, Cl, Br, etc.) have 7 valence electrons. The noble gases in the right-most column (Ne, Ar, Kr, etc.) all have 8 electrons in their outermost electron shell except for He, which only has 2 electrons. Periods ''' The horizontal rows of the periodic table are called periods. These are named and numbered in the same way as first period, second period, and so on. Periods do not share specific characteristics, but are important terminology when addressing the trends of the elements within the table. '''Trends The properties of the atoms for each element demonstrate trends in ionization energy, electron affinity and atomic size, that can be applied to all the elements in the table. Ionization energy Ionization energy is the energy that is needed to remove an electron from an atom in the gaseous state. Although there may be some exceptions, in general: :::As you move across a period from left to right, the ionization energy increases. '' :::''As you move down a group, ionization energy decreases . Electron affinity Electron affinity is the energy change associated with the addition of an electron to a gaseous atom. This is usually the energy that is being released when an electron is being added. The more negative the energy, the greater this amount of energy is. :::Electron affinities become more negative, as you move across a period from left to right. '' There are many exceptions to the general trend associated with electron affinity ::: As you move down a group, electron affinity becomes more positive.'' However, the changes in electron affinity down a group is usually so small that there are even more exceptions to this rule. Atomic radius Atomic radius refers to the size of the atom. The more specific definition of the atom’s radius is half the distance between the nuclei in a molecule of identical atoms. :::Atomic radii decrease, as you move left to right along the period of the table. This decrease is due to the decrease in shielding that exists from left to right. The valence electrons are drawn closer to the nucleus of the atom, decreasing the sixe of the atom. :::Atomic radius increases, as you move down a group. References *Zumdahl, Steven S. (2000). Chemistry (5th ed.). Boston, MA.: Houghton Mifflin Company.